This invention relates to an automatic exposure control device for endoscope of a mean photometry type.
An endoscope is generally used for observations inside living body cavities or precision machines. The endoscope is composed of an objective section, an eyepiece section, and a coupling section for optically connecting these two sections. A light source for forming a subject image by means of an objective of the objective section may be disposed externally and coupled to the objective section by means of a light guide formed of glass fiber, or disposed directly inside the objective section. The subject image provided by the objective is transmitted to an eyepiece through an image transmitting optical system, and observed. In photographing the subject image, a camera has usually been attached to the eyepiece section for the convenience of observation. When using an expensive fiber scope in photographing, the eyepiece section is provided with a light sensing element for each unit of endoscope, and a signal is sent to the light source unit in accordance with a quantity of light passed through the eyepiece section, and the light supply to the light source unit is controlled automatically when a proper exposure value is reached. When using a low-priced rigid endoscope, on the other hand, the light sensing element is not provided for each unit of endoscope, and the exposure control is conducted manually by experience. Thereupon, the endoscope is subject to different thicknesses of the image transmitting optical system and different magnifications of the eyepiece depending on a applications. That is, the size of the subject image on a film surface is generally smaller than that of the film surface due to the effect of brightness, varying with the type of endoscope. As a result, in a photometer circuit of a mean photometry type, the photometric quantity (detected by the light sensing element) varies with the size of the subject image even though the same illuminance is obtained for the unit area on the film surface. If the subject image appears only on part of the film surface, for example, the quantity of light sensed will be levelled to provide a photometric quantity level lower than the actual level of illumination.
Namely, the use of a narrow endoscope (where the subject image on the film is small) causes overexposure. Accordingly, when using endoscopes of different thicknesses in combination with a photographing/light source unit capable of automatic exposure control (in accordance with the quantity of light sensed), the exposure control circuit must be supplied with correction information in accordance with the thickness of the endoscope (size of subject image). However, there are so many types of endoscopes that it is quite troublesome and mistakable for a user to switch the automatic exposure control circuit of the camera selectively in accordance with the thickness of the endoscope every time he uses the device. Moreover, the manual switching of the automatic exposure control circuit by the user may halve the advantages of automatic exposure.